Method of heating glass sheet for laminated glass

ABSTRACT

First and second glass sheets, which are to be combined together into a single laminated glass sheet, are heated in a heating furnace so that the temperature of the first glass sheet at the exit of the heating furnace and the temperature of the second glass sheet at the exit of the heating furnace are substantially equal to each other.

This is a division, of application Ser. No. 07/525,537, filed May 18,1990 now U.S. Pat. No. 5,074,900.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet glass heating method, and moreparticularly to a method of heating a glass sheet for laminated glass.

2. Description of the Relevant Art

Laminated glass, which finds wide use as front windshields ofautomobiles, is manufactured by heating first and second glass sheets tobe paired in a heating furnace, shaping and annealing the glass sheets,and then joining the glass sheets face to face. The glass sheets may beshaped by a press shaping process, a gravity shaping process, or aroller shaping process. The first and second glass sheets are joinedface to face by adhesive bonding with an intermediate film as ofpolyvinyl butyral sandwiched therebetween.

When the first and second glass sheets are to be bonded to each other,it is necessary that they be of a bent configuration havingsubstantially the same shape. Basically, the first and second glasssheets are heated under the same conditions in the heating furnace sothat they are held at the same temperature To at the exit of the heatingfurnace. The first and second glass sheets are fed, alternately one byone, two by two, or three by three, into the heating furnace.

The first and second glass sheets, from which laminated glass is to beconstructed, may not necessarily have the same characteristics, such asthicknesses, material properties, and colored conditions, at all times.For example, the first and second glass sheets may have differentthicknesses, or the first glass sheet may be a colored transparent glasssheet whereas the second glass sheet may be a colorless transparentglass sheet. If the first and second glass sheets have such differentcharacteristics, then they tend to be held at different temperatures Toat the exit of the heating furnace. As a result, the first and secondglass sheets may not be shaped desirably, and may not appropriately bebonded to each other after they are shaped and annealed.

The present invention has been made in an effort to effectively solvethe aforesaid problems of the conventional method of heating glasssheets for laminated glass.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of heatingfirst and second glass sheets for laminated glass so that even if thefirst and second glass sheets have different characteristics, they canbe held at the same temperature To at the exit of a heating furnace, canbe shaped to a desired configuration, and can as well be bonded face toface to each other after they are shaped and annealed.

To achieve the above object, there is provided in accordance with thepresent invention a method of heating a first glass sheet and a secondglass sheet which are to be combined into a single laminated glasssheet, in a heating furnace before the first and second glass sheets areshaped, the first glass sheet being heatable more easily than the secondglass sheet, the method comprising the step of heating the first andsecond glass sheets so that the temperature of the first glass sheet atthe exit of the heating furnace and the temperature of the second glasssheet at the exit of the heating furnace are substantially equal to eachother.

The above and further objects, details and advantages of the presentinvention will become apparent from the following detailed descriptionof preferred embodiments thereof, when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a sheet glass bending apparatus whichcarries out a method of heating glass sheets for laminated glassaccording to a first embodiment of the present invention;

FIG. 2 is a fragmentary longitudinal cross-sectional view of a heatingfurnace of the sheet glass bending apparatus shown in FIG. 1;

FIGS. 3A through 3C are views showing various sequences by which firstand second glass sheets are heated;

FIG. 4 is a schematic plan view of a sheet glass bending apparatus whichcarries out a method of heating glass sheets for laminated glassaccording to a second embodiment of the present invention;

FIG. 5 is a schematic plan view of a sheet glass bending apparatus whichcarries out a method of heating glass sheets for laminated glassaccording to a third embodiment of the present invention;

FIG. 6 is a fragmentary longitudinal cross-sectional view of an upstreamportion of the sheet glass bending apparatus shown in FIG. 5;

FIGS. 7 and 8 are perspective views of heaters for use in a preheatingzone in the sheet glass bending apparatus shown in FIG. 5;

FIG. 9 is a fragmentary longitudinal cross-sectional view of a sheetglass bending apparatus which carries out a method of heating glasssheets for laminated glass according to a fourth embodiment of thepresent invention;

FIG. 10 is a fragmentary longitudinal cross-sectional view of a sheetglass bending apparatus which carries out a method of heating glasssheets for laminated glass according to a fifth embodiment of thepresent invention;

FIG. 11 is a transverse cross-sectional view of a sheet glass bendingapparatus which carries out a method of heating glass sheets forlaminated glass according to a sixth embodiment of the presentinvention;

FIG. 12 is a fragmentary longitudinal cross-sectional view of a sheetglass bending apparatus which carries out a method of heating glasssheets for laminated glass according to a seventh embodiment of thepresent invention; and

FIG. 13 is a graph showing the relationship between heating times andtemperatures at the exit of a heating furnace when various glass sheetsare heated in the heating furnace which is kept at a constanttemperature therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 13 shows the relationship between heating times and glasstemperaturesat the exit of a heating furnace when four kinds of sheetglass are heated in the heating furnace which is kept at a constanttemperature Ti therein.All the four glass sheets are sized 450 mm×450mm. The horizontal axis of the graph of FIG. 13 represents a heatingtime t and the vertical axis represents the glass temperature To at theexit of the heating furnace. The characteristics of the heated glasssheets are indicated by respective curves X1, X2, Y1, Y2.

In the experiment to obtain the data shown in FIG. 13, the heatingfurnace had ceramic rollers for feeding glass sheets and electricheaters inside of the furnace wall. The surface temperature of each ofthe heaters was controlled so as to be at a desired level by athermocouple. Specifically,the surface temperature of the heaters abovethe rollers was kept at 660° C., and the surface temperature of theheaters below the rollers was kept at 650° C.

    ______________________________________                                        Thickness  Color                                                              ______________________________________                                        X1: 2.0 mm Blue (= colored transparent glass)                                 X2: 2.3 mm Blue (= colored transparent glass)                                 Y1: 2.0 mm Colorless (= colorless transparent glass)                          Y2: 2.3 mm Colorless (= colorless transparent glass)                          ______________________________________                                    

It can be understood from FIG. 13 that when the glass sheets are heatedunder the same conditions, the temperatures To of the colored glasssheetsat the exit of the heating furnace are generally higher than thetemperatures To of the colorless glass sheets at the exit of the heatingfurnace. Comparison between the colored glass sheets or the colorlessglass sheets indicates that the glass temperature To at the heatingfurnace exit is higher if the glass sheet thickness is smaller.

If the shape, material property, or color of a first glass sheet forlaminated glass is different from the shape, material property, or colorof a second glass sheet, therefore, when the first and second glasssheetsare heated under the same conditions, their temperatures To at theexit of the heating furnace are different from each other.

FIG. 1 schematically shows a sheet glass bending apparatus, generallydesignated by the reference numeral 100, which carries out a method ofheating glass sheets for laminated glass according to a first embodimentof the present invention. It is assumed that a single laminated glasssheet is constructed of first and second glass sheets G1, G2 and thefirstglass sheet G1 can be heated more easily than the second glasssheet G2.

The sheet glass bending apparatus 100 comprises a heating furnace 1which is heated to a constant temperature Ti therein, a press machine 2disposeddownstream of the heating furnace 1, a lehr 3 disposeddownstream of the press machine 2, and a pickup device 4 disposeddownstream of the lehr 3. A succession of first rollers 5 (FIG. 2),serving as a feed conveyor, is disposed in an upstream region in theheating furnace 1, for feeding the first and second glass sheets G1, G2into the heating furnace 1. The firstrollers 5 are arrayed over adistance L1 along the direction in which glasssheets are fed through theheating furnace 1. Another succession of second rollers 6, also servingas a conveyor, is disposed in the heating furnace 1, following the firstrollers 5. The second rollers 6 are arrayed over a distance L3 along theglass sheet feeding direction. As seen in FIGS. 1 and 2, the first andsecond rollers 5, 6 continuously feed the first and second glass sheetsG1, G2 within the furnace 1, while keeping the glass sheets G1, G2horizontal. The first rollers 5 are rotatable selectively atdifferentspeeds such that they can feed glass sheets at a higher speed Vh and arelatively low constant speed V1. The second rollers 6 are howeverrotatable at a fixed speed such that they can feed glass sheets at therelatively low constant speed V1.

Initially, a single first glass sheet G1 is fed into the heating furnace1 by the first rollers 5 and displaced over the distance L1 at thehigher speed Vh. Then, the glass sheet G1 is delivered over the distanceL3 at the lower speed V1 by the second rollers 6.

When the glass sheet G1 has traversed the distance L1, a second glasssheetG2 is introduced into the heating furnace 1 by the first rollers 5.At thistime, the introduced second glass sheet G2 is fed over a distanceL2, shorter than the distance L1, at the higher speed Vh. Then, thespeed at which the second glass sheet G2 is fed is shifted from thehigher speed Vhto the lower speed V1 by a control mechanism (not shown).The second sheet glass G2 is subsequently fed over a distance L4 at thelower speed V1, thedistance L4 satisfying the relationship: L1+L3=L2+L4.

From the exit or terminal end of the heating furnace 1, the glass sheetsG1, G2 are successively sent to the press machine 2 where they arepressedto shape.

When the first and second glass sheets G1, G2 are heated in the abovefashion, the temperature To1 of the first glass sheet G1 at the exit ofthe heating furnace and the temperature To2 of the second glass sheet G2at the heating furnace exit are equalized to each other. The ratiobetweenthe distances L1, L3, the ratio between the distances L2, L4, andthe specific values of the speeds Vh, V1 are determined depending on thepreset temperature Tin in the heating furnace 1 and desired temperaturesTo1, To2 of the glass sheets G1, G2 at the exit of the heating furnace1.

Preferably, first and second glass sheets G1, G2 are introduced into theheating furnace 1 such that glass sheets G1, G2 to be paired into alaminated glass sheet are disposed closely to each other. When the glasssheets G1, G2 are heated in the manner described above, they arealternately introduced into the heating furnace 1 as shown in FIG. 3A,andsuccessive first and second glass sheets G1, G2 are combined intolaminatedglass sheets.

As shown in FIG. 3B, sets of two first glass sheets G1 and sets of twosecond glass sheets G2 may alternately be introduced into the heatingfurnace 1, and successive two glasses G1, G2 or successive two glassesG2,G1 may be combined into laminated glass sheets.

Alternatively, as shown in FIG. 3C, sets of three first glass sheets G1andsets of three second glass sheets G2 may alternately be introducedinto theheating furnace 1, and each of the first glass sheets G1 in oneset may be combined with the third following second glass sheet G2 inthe next set, thereby making up a laminated glass sheet.

In the sheet glass bending apparatus 100, the first and second glasssheetsG1, G2 are fed over the same distance (=L1+L3=L2+L4) in theheating furnace1, but the average speed at which the first glass sheetG1 that can be heated relatively easily is fed in the heating furnace 1is higher than the average speed at which the second glass sheet G2 isfed in the heatingfurnace 1.

FIG. 4 schematically shows a sheet glass bending apparatus, generallydesignated by the reference numeral 200, which carries out a method ofheating glass sheets for laminated glass according to a secondembodiment of the present invention. Those components shown in FIG. 4which are identical to those shown in FIG. 1 are denoted by theidentical reference numerals, and will not be described in detail.

The sheet glass bending apparatus 200 comprises a heating furnace 201whichis heated to a constant temperature Ti therein, a press machine 2,a lehr 3, and a pickup device 4 which are successively disposeddownstream of theheating furnace 201.

Although not shown, the second rollers 6 are disposed in an upstreamregionin the heating furnace 1 and arrayed over the distance L3, and thefirst rollers 5 are disposed downstream of the second rollers 6 andarrayed overthe distance L1 in the heating furnace 1.

In operation, a single first glass sheet G1 is introduced into theheating furnace 201 by the second rollers 6 and fed over the distance L3at the lower speed V1. Then, the glass sheet G1 is fed over the distanceL1 at the higher speed Vh by the first rollers 5 until the glass sheetG1 reaches the exit at the terminal end of the heating furnace 201.

While the glass sheet G1 is being fed over the distance L3, a secondglass sheet G2 is introduced into the heating furnace 201 by the secondrollers 6. The glass sheet G1 is fed over the distance L4 at the lowerspeed V1 first by the second rollers 6 and then by the first rollers 5.The second glass sheet G2 is further fed over the distance L2 by thefirst rollers 5 before it arrives at the exit at the terminal end of theheating furnace 201.

The glass sheets G1, G2 are successively fed from the terminal end ofthe heating furnace 201 to the next press machine 2, by which the glasssheetsG1, G2 are pressed to shape.

In the heating method described above with reference to FIG. 4, thetemperature To1 of the first glass sheet G1 at the exit of the heatingfurnace and the temperature To2 of the second glass sheet G2 at theheating furnace exit are equalized to each other. More specifically, inthe sheet glass bending apparatus 200, the first and second glass sheetsG1, G2 are fed over the same distance (=L3+L1=L4+L2) in the heatingfurnace 201, but the average speed at which the first glass sheet G1that can be heated relatively easily is fed in the heating furnace 201is higher than the average speed at which the second glass sheet G2 isfed inthe heating furnace 201.

In the heating furnaces 1, 201 shown in FIGS. 1 and 4, respectively, thefirst rollers 5 which are rotatable selectively at different speeds aredisposed in one of the upstream and downstream regions in the heatingfurnace. However, the first rollers 5 may be disposed so as to existover the entire length of the heating furnace, and may be controlled soas to rotate selectively at different speeds.

The heating methods to be carried out by the sheet glass bendingapparatus 100, 200 have to meet only the one requirement that theaverage speed at which the first glass sheet G1 is fed in the heatingfurnace be higher than the average speed at which the second glass sheetG2 is fed in the heating furnace. Therefore, with the lower speed V1 andthe higher speed Vh being set to suitable values, only the first glasssheet G1 may be fed at the higher speed through a portion of the heatingfurnace and the second glass sheet G2 may be fed at the lower speed overthe entire lengthof the heating furnace.

FIGS. 5 and 6 show a sheet glass bending apparatus, generally designatedbythe reference numeral 300, which carries out a method of heating glasssheets for laminated glass according to a third embodiment of thepresent invention. Those components shown in FIGS. 5 and 6 which areidentical to those shown in FIG. 1 are denoted by the identicalreference numerals, andwill not be described in detail.

The sheet glass bending apparatus 300 comprises a heating furnace 301whichis heated to a constant temperature Ti therein, a preheating zone307 positioned upstream of the heating furnace 301, an intermediate zone308 disposed between the heating furnace 301 and the preheating zone307, and a press machine 2, a lehr 3, and a pickup device 4 which aresuccessively disposed downstream of the heating furnace 301. The secondrollers 6 are disposed in the preheating zone 307, the intermediate zone308, and the heating furnace 301, the second rollers 6 being arrayedover the entire length of the heating furnace 301. The preheating zone307 has line burners 309 (see FIG. 7) disposed above and below a glasssheet feed path,for heating the upper and lower surfaces of second glasssheets G2. The preheating zone 307 may have a panel heater 310 (see FIG.8) instead of the line burners 309. The intermediate zone 308 may bepart of the heatingfurnace 301.

A first glass sheet G1 is introduced at ambient temperature through theintermediate zone 308 into the heating furnace 301.

A second glass sheet G2 is first introduced into the preheating zone 307inwhich the upper and lower surfaces of the second glass sheet G2 areuniformly heated by the burners 309 until the glass sheet G2 reaches apredetermined temperature in its entirety. The intensities of heatappliedto the upper and lower surfaces of the glass sheet G2 are thus soequalizedthat the glass sheet G2 is prevented from warping when it ispreheated. Thereafter, the second glass sheet G2 is introduced into theheating furnace 301.

After the first and second glass sheets G1, G2 have been introduced intothe heating furnace 301, they are fed over the entire length of theheating furnace 301 at the lower speed V1 by the second rollers 6. Thesecond glass sheet G2 is introduced into the heating furnace 301,following the first glass sheet G1. Therefore, the glass sheets G1, G2areintroduced into the heating furnace 301 in the sequence, and combinedtogether in the pattern, shown in FIG. 3A. Of course, the first andsecondglass sheets G1, G2 may be introduced in the sequences, andcombined in thepatterns, shown in FIGS. 3B and 3C.

When the glass sheets G1, G2 are thus heated, the temperature To1 of thefirst glass sheet G1 at the exit of the heating furnace and thetemperature To2 of the second glass sheet G2 at the heating furnace exitare equalized to each other. More specifically, in the sheet glassbendingapparatus 300, the distances over, and the average speeds at,which the first and second glass sheets G1, G2 are fed in the heatingfurnace 301, are equal to each other. However, the temperatures To1, To2are equalized to each other since the second glass sheet G2 is preheatedbefore it is introduced into the heating furnace 301.

The degree to which the second glass sheet G2 is preheated in thepreheating zone 307 is determined depending on the temperature Ti in theheating furnace Ti and desired temperatures To1, To2 of the glass sheetsG1, G2 at the exit of the heating furnace 301.

FIGS. 9, 10, 11, 12 schematically show sheet glass bending apparatus,generally designated by the reference numerals 400, 500, 600, 700,respectively, which carries out methods of heating glass sheets forlaminated glass according to fourth, fifth, sixth, and seventhembodimentsof the present invention.

In each of the sheet glass bending apparatus 400, 500, 600, 700, thesecondrollers 6 are disposed in and arrayed over the entire length ofthe heatingfurnace. Therefore, the first and second glass sheets G1, G2are fed over the same distance at the same average speed in the heatingfurnace. However, the intensity of heat radiated to the first glasssheet G1 in theheating furnace is made lower than the intensity of heatradiated to the second glass sheet G2 in the heating furnace in themanner described below. As a result, in each of the sheet glass bendingapparatus 400, 500,600, 700, the temperature To1 of the first glasssheet G1 at the exit of the heating furnace and the temperature To2 ofthe second glass sheet G2 at the heating furnace exit are equalized toeach other.

In the sheet glass bending apparatus 400 shown in FIG. 9, a series ofgas burners 411 which can adjust the pressure of a supplied gas aredisposed in a heating furnace 401. The pressure of a gas supplied toeach of the gas burners 411 is adjusted to a certain level such that thegas burners 411 radiate a certain intensity of heat.

First, a first glass sheet G1 is introduced into the heating furnace401, and fed by the rollers 6. While the glass sheet G1 is being fed inthe heating furnace 401, the pressure of a gas supplied to the gasburners 411is lowered by a control mechanism (not shown) thereby toreduce the quantity of heat radiated to the glass sheet G1. Afterpassage of the glass sheet G1, the lowered gas pressure is increasedagain. Therefore, asthe glass sheet G1 is progressively fed through theheating furnace 401, the gas pressures of the gas burners 401 aresuccessively lowered and increased again. Following the first glasssheet G1, a second glass sheet G2 is introduced into and fed through theheating furnace 401 in which it is heated.

In the sheet glass bending apparatus 500 shown in FIG. 10, a series ofgas burners 511 which are vertically movable between lifted and loweredpositions are disposed in a heating furnace 501. The gas burners 511 areusually held in the lowered position.

First, a first glass sheet G1 is introduced into the heating furnace501, and fed by the rollers 6. While the glass sheet G1 is being fed inthe heating furnace 501, the gas burners 511 are elevated to the liftedposition by a control mechanism (not shown) thereby to reduce theintensity of heat radiated to the glass sheet G1. After passage of theglass sheet G1, the elevated gas burners 511 are lowered to the loweredposition. Therefore, as the glass sheet G1 is progressively fed throughthe heating furnace 501, the gas burners 501 are successively elevatedandlowered again. Following the first glass sheet G1, a second glasssheet G2 is introduced into and fed through the heating furnace 501 inwhich it is heated.

In the sheet glass bending apparatus 600 shown in FIG. 11, a series ofgas burners 611 which are swingable laterally across the glass sheetfeed pathare disposed in a heating furnace 601. The gas burners 611 areusually directed downwardly.

First, a first glass sheet G1 is introduced into the heating furnace601, and fed by the rollers 6. While the glass sheet G1 is being fed inthe heating furnace 601, the gas burners 611 are swung laterally by acontrol mechanism (not shown) thereby to reduce the intensity of heatradiated to the glass sheet G1. After passage of the glass sheet G1, thegas burners 611 are returned to the downwardly oriented position.Therefore, as the glass sheet G1 is progressively fed through theheating furnace 601, the gas burners 611 are successively swunglaterally and returned to the downwardly oriented position again.Following the first glass sheet G1, a second glass sheet G2 isintroduced into and fed through the heating furnace 601 in which it isheated.

In the sheet glass bending apparatus 700 shown in FIG. 12, a series ofgas burners 711 are disposed in a heating furnace 701. The heatingfurnace 701houses a horizontal shield plate 712 disposed between theburners 711 and the rollers 6, the shield plate 712 being horizontallymovable reciprocally along the glass sheet feed path in the heatingfurnace 701.

First, a first glass sheet G1 is introduced into the heating furnace701, and fed by the rollers 6. While the glass sheet G1 is being fed inthe heating furnace 701, the shield plate 712 is moved with the glasssheet G1over a predetermined distance by a control mechanism (not shown)such that the shield plate 712 is positioned upwardly of the glass sheetG1. A second glass sheet G2 is introduced into the heating furnace 701,following the first glass sheet G1, and is heated while being fed in theheating furnace 701.

While the gas burners are illustrated in the above embodiments shown inFIGS. 9 through 12, other heating means such as electric heating wiresor hot air may be used in place of the gas burners.

With the heating methods according to the present invention, asdescribed above, the temperatures To1, To2, at the exit of the heatingfurnace, of first and second glass sheets G1, G2 which are to becombined into a laminated glass sheet and which have differentcharacteristics are equalized to each other. Therefore, the glass sheetsG1, G2 can be pressedor otherwise processed into a desired shape. As aconsequence, the glass sheets G1, G2 can well be bonded to each otherafter they have been shapedand annealed.

In the illustrated embodiments, it is necessary that the temperaturesTo1, To2 of the first and second glass sheets G1, G2 at the exit of theheatingfurnace be equal to each other. However, this requirement may notbe satisfied if the glass sheets G1, G2 should be heated to differenttemperatures To1, To2 at the exit of the heating furnace depending onthe condition in which the glass sheets G1, G2 will subsequently be bentto shape.

Although there have been described what are at present considered to bethepreferred embodiments of the present invention, it will be understoodthat the invention may be embodied in other specific forms withoutdeparting from the essential characteristics thereof. The presentembodiments are therefore to be considered in all aspects asillustrative, and not restrictive. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription.

We claim:
 1. A method of heating a first glass sheet and a second glass sheet, which are to be combined into a single laminated glass sheet, in a constant temperature heating furnace before the first and second glass sheets are shaped, the first glass sheet being heatable more easily than the second glass sheet, said method comprising the steps of:(a) preheating the second glass sheet before the second glass sheet is introduced into the heating furnace; (b) introducing into the heating furnace, the first glass sheet at ambient temperature and the second glass sheet at a preheated temperature; and (c) heating the first and second glass sheets while the first glass sheet is continuously being fed over the same distance in the heating furnace so that the temperatures of the first glass sheet and the second glass sheet at the exit of the heating furnace are substantially equal to each other.
 2. A method of heating a first glass sheet and a second glass sheet which are to be joined into a laminated glass sheet, in a heating furnance, wherein the first glass sheet can be heated more easily than the second glass sheet, said method comprising the steps of:(a) preheating the second glass sheet to a predetermined temperature; and (b) introducing into the heating furnace, the first glass sheet at ambient temperature and the preheated second glass sheet at the predetermined temperature.
 3. A method of heating glass sheets for laminated glass, comprising the steps of:(a) preheating a second glass sheet positioned upstream from a heating furnace in which upper and lower surfaces of the second glass sheet are uniformly preheated until the second glass sheet reaches a predetermined temperature in its entirety; (b) introducing a first glass sheet at ambient temperature and the preheated second glass at the predetermined temperature into the heating furnace; and (c) feeding the first and second glass sheets over the entire length of the heating furnace.
 4. The method as set forth in claim 3 above, wherein the preheating step (a) further comprises the step of equalizing the intensities of heat applied to the upper and lower surfaces of the second glass sheet so that the second glass sheet is prevented from warping.
 5. The method as set forth in claim 3 above, wherein the introducing step (b) is sequenced to create a pattern selected from any one of FIGS. 3A, 3B or 3C.
 6. The method as set forth in claim 3 above, wherein the temperatures of the first glass sheet and the second glass sheet at the exit of the heating furnace are substantially equal to each other.
 7. The method as set forth in claim 3 above, wherein the average speeds at which the first and second glass sheets are fed over the length of the heating furnace are substantially equal to each other.
 8. The method as set forth in claim 3 above, wherein the predetermined temperature in the preheating step (a) is predetermined by the steps of:(i) determining the temperature in the heating furnace; and (ii) determining the desired temperatures of the first and second glass sheets at the exit of the heating furnace. 